Communication protocols rely on various routing techniques to transfer data between communication endpoints on a communication network. Communication or network protocols and the corresponding routing strategies are typically selected in view of such factors as knowledge of network topology, size of the network, type of medium used as a signal carrier, security and reliability requirements, tolerable transmission delays, and types of devices forming the network. Due to a large number of such factors, a typical routing technique meets some of the design objectives at the expense of the others. For example, a certain routing technique may provide a high level of reliability in data delivery but may also require a relatively high overhead. Thus, while there are many known approaches to routing and many protocols compatible with these routing methods, there remain communication networks with the specific requirements that are not fully satisfied by any of the available routing methods and protocols. Moreover, as new types of communication networks, with the increasing demands for efficiency, throughput, and reliability, emerge in various industrial and commercial applications, the architects and developers frequently encounter new problems which are not easily addressed by the existing protocols and the associated routing techniques.
Generally speaking, a communication network includes nodes which are the senders and recipients of data and communication paths connecting the nodes. Additionally, communication networks typically include dedicated routers responsible for directing traffic between nodes, and, optionally, dedicated devices responsible for configuring and managing the network. Some or all of the nodes may be also adapted to function as routers in order to direct traffic sent between other network devices. Network devices may be inter-connected in a wired or wireless manner, and network devices may have different routing and transfer capabilities. For example, dedicated routers may be capable of high volume transmissions while some nodes may be capable of sending and receiving relatively little traffic over the same period of time. Additionally, the connections between nodes on a network may have different throughput capabilities and different attenuation characteristics. A fiberoptic cable, for example, may be capable of providing a bandwidth several orders of magnitude higher than a wireless link because of the difference in the inherent physical limitations of the medium.
In order for a node to send data to another node on a typical network, either the complete path from the source to the destination or the immediately relevant part of the path must be known. For example, the World Wide Web (WWW) allows pairs of computer hosts to communicate over large distances without either host knowing the complete path prior to sending the information. Instead, hosts are configured with the information about their assigned gateways and dedicated routers. In particular, the Internet Protocol (IP) provides network layer connectivity to the WWW. IP defines a sub-protocol known as Address Resolution Protocol (ARP) which provides a local table at each host specifying the routing rules. Thus, a typical host connected to the WWW or a similar Wide Area Network (WAN) may know to route all packets with the predefined addresses matching a pre-configured pattern to host A and route the rest of the packets to host B. Similarly, the intermediate hosts forwarding the packets, or “hops,” also execute partial routing decisions and typically direct data in the general direction of the destination.
Routing strategies on a typical network may be further complicated by scheduling issues. In general, scheduling refers to allocation of resources, such as timeslots on a wired or wireless link, to devices participating in communications on a network. Selecting a proper scheduling strategy and generating the optimal schedule for a particular network may be particularly relevant in a wireless environment. Because the number of available frequencies is typically limited, network hosts may not be able to transmit or receive data as soon as this data becomes available. For example, a pair of communicating devices, each capable of operating in receive and transmit modes, may exchange data over a single carrier frequency. In order to resolve potential collisions during transmissions and prevent the devices from missing data by failing to enter the receive mode at the right time, one could define a schedule assigning some transmission opportunities to the first device and the rest of the transmission opportunities to the second device. By complying with the schedule, the pair of devices could successfully maintain bidirectional data exchange over the same carrier frequency.
Unlike the example discussed above, most wireless networks include numerous devices and each device may have idiosyncratic requirements with respect to the amount of data the device needs to transmit, the rates of transmission and reception, the maximum amount of data the device is capable of receiving and transmitting per unit of time, the tolerable latency and potentially many other factors. Thus, scheduling decisions may become very complex and optimizing scheduling may become a high priority in many applications. Moreover, routing decisions and scheduling decisions may have a significant impact on each other and, as a result, may require an even more complicated simultaneous definition and optimization.
In short, there is a large number of factors influencing the implementation of particular protocols in particular industries. In the process control industry, it is known to use standardized communication protocols to enable devices made by different manufacturers to communicate with one another in an easy to use and easy to implement manner. One such well known communication standard used in the process control industry is the Highway Addressable Remote Transmitter (HART) Communication Foundation protocol, referred to generally as the HART protocol. Generally speaking, the HART protocol supports a combined digital and analog signal on a dedicated wire or set of wires, in which on-line process signals (such as control signals, sensor measurements, etc.) are provided as an analog current signal (e.g., ranging from 4 to 20 milliamps) and other signals, such as device data, requests for device data, configuration data, alarm and event data, etc., are provided as digital signals superimposed or multiplexed onto the same wire or set of wires as the analog signal. However, the HART protocol currently requires the use of dedicated, hardwired communication lines, resulting in significant wiring needs within a process plant.
There has been a move, in the past number of years, to incorporate wireless technology into various industries including, in some limited manners, the process control industry. However, there are significant hurdles in the process control industry that limit the full scale incorporation, acceptance and use of wireless technology, as the process control industry requires a completely reliable process control network because loss of signals can result in the loss of control of a plant, leading to catastrophic consequences, including explosions, the release of deadly chemicals or gases, etc. For example, Tapperson et al., U.S. Pat. No. 6,236,334 discloses the use of a wireless communications in the process control industry as a secondary or backup communication path or for use in sending non-critical or redundant communication signals. Moreover, there have been many advances in the use of wireless communication systems in general that may be applicable to the process control industry, but which have not yet been applied to the process control industry in a manner that allows or provides a reliable, and in some instances completely wireless, communication network within a process plant. U.S. Patent Application Publication Numbers 2005/0213612, 2006/0029060 and 2006/0029061 for example disclose various aspects of wireless communication technology related to a general wireless communication system.
Similar to wired communications, wireless communication protocols are expected to provide efficient, reliable and secure methods of exchanging information. Of course, much of the methodology developed to address these concerns on wired networks does not apply to wireless communications because of the shared and open nature of the medium. Further, in addition to the typical objectives behind a wired communication protocol, wireless protocols face other requirements with respect to the issues of interference and co-existence of several networks that use the same part of the radio frequency spectrum. Moreover, some wireless networks operate in the part of the spectrum that is unlicensed, or open to the public. Therefore, protocols servicing such networks must be capable of detecting and resolving issues related to frequency (channel) contention, radio resource sharing and negotiation, etc.
In order to properly configure a wireless network, engineers and maintenance personnel must consider a large number of factors. In particular, engineers must evaluate at least the topology of the network and the capacity of network connections. Moreover, many applications in the process control industry, to take one example, require a degree of reliability, security, and efficiency which is significantly higher than the standards applied to most commercial or household applications. In order to meet these additional requirements, process control engineers must optimize both routing and scheduling in the wireless network. In other words, engineers must simultaneously pursue several design objectives, such as reducing latency, increasing reliability, and minimizing cost. Some of these objectives may not be compatible with each other at all times and the engineers may have to make difficult trade-off decisions. In cases when large plants have process control networks including many devices of different types, efficiently designing a wireless network may become even more time-consuming and challenging. Meanwhile, even minor mistakes in configuration may noticeably reduce the efficiency of a plant in which a wireless process control network is implemented and thus cause operators to incur significant financial losses.
Further, new facts or design considerations may become apparent only during the operation of a wireless network. For this reason, engineers may require a certain amount of testing prior to deployment. One or more tests may generate new data, parameters, and measurements which must then be incorporated into the existing design and, in particular, into the previously developed routes and schedules. Efficiently applying test data to an existing configuration without re-designing the entire network may become a challenging technical issue comparable to the difficulty of creating the original design.
Still further, network nodes may be added, removed, or repositioned in an existing wireless network, thereby rendering some of the routing and scheduling schemes ineffective or deficient. To continue with the example of the process control industry, a change in a network layout may require a new network configuration and a possible shutdown of a plant for the duration of configuration and testing.